1. Field of the Invention
The present invention relates to a motor driving device, an image forming apparatus, and a motor driving method.
2. Description of the Related Art
Currently, direct current (DC) brushless motors are widely used for operation of equipment that requires high operating precision.
For example, a DC brushless motor is used as a driving source for rotating a photosensitive drum in an electrophotographic image forming apparatus. In such an image forming apparatus, a laser diode (LD) is ON/OFF controlled by image data, the photosensitive drum is exposed to main/sub two-dimensional scanning of generated light beams to write an image thereon. In this writing operation, the rotation of the photosensitive drum is responsible for sub-scanning. If the rotation of the photosensitive drum varies, a positional shift occurs in part of the image on a main scanning line, resulting in reduced image quality. In the case of a color image, write operation is performed for each color, whereby a color shift occurs unless the speed is maintained constant. In view of the foregoing, a DC brushless motor driving device capable of maintaining a specified speed at high precision is required in driving the photosensitive drum.
For example, Japanese Patent Application Laid-Open No. 2006-6066 has proposed a motor driving device that controls the drive of a DC brushless motor. The conventional motor driving device includes a control circuit that performs digital control and a motor drive circuit that drives the motor in response to a command from the control circuit. In the conventional motor driving device, a frequency generator (FG) signal corresponding to the rotation frequency of the motor generated in an FG signal generating unit is amplified in the motor drive circuit (driver), and fed back to the control circuit (control application-specific integrated circuit (ASIC)) after being analog-to-digital (A/D) converted. The control circuit digitally calculates a current control amount with respect to each phase of the DC brushless motor for achieving a target speed based on the FG signal. The current control amount is input to the motor drive circuit as a pulse width modulation (PWM) signal so that the DC brushless motor is driven at the target speed.
In the following, a motor driving device is explained that includes a DC brushless motor as a main motor of an image forming apparatus similarly to the above conventional motor driving device. FIG. 12 is a block diagram of the motor driving device. FIG. 13 is a schematic diagram of signal waveforms generated in circuits.
The motor driving device includes a DC brushless motor as a main motor 1, a control substrate 12 including a motor control circuit, and a drive substrate 13 including a motor drive circuit. The drive substrate 13 is not arranged above the control substrate 12, but is arranged above another substrate or as an independent substrate.
The control substrate 12 includes thereon an ASIC 11, a FG filter 4, and a filter 8 corresponding to the motor control circuit. The ASIC 11 is capable of receiving and outputting digital signals, and performs digital processing.
An FG sensor 14 includes a multipole magnetized rotor magnet of the main motor 1 and a rectangular coil pattern that is arranged circularly to face a magnetized surface of the rotor magnet. When the rotor magnet rotates at the time of driving the main motor 1, a voltage is induced in the coil pattern, and the FG sensor 14 outputs a sine wave signal with a frequency corresponding to the rotation speed of the main motor 1 as indicated by a signal waveform of (a) in FIG. 13. The FG sensor output corresponds to an analog rotation frequency (FG) signal.
The FG sensor output is a weak signal, and thus is amplified by an FG amplifier 2 on the drive substrate 13. The FG sensor output is then converted to a digital signal with a rectangular waveform by a Schmitt comparator 3 of the motor drive circuit, and output to the control substrate 12 as a feed back signal. In FIG. 13, (b) indicates the output of the FG amplifier 2 after amplification, and (c) indicates the rectangular waveform obtained by A/D converting the amplified analog FG signal of (b) with a threshold value in the Schmitt comparator 3.
The FG sensor output input to the control substrate 12 is influenced by external noise on a transmission path from the drive substrate 13 arranged on a separate circuit substrate to the control substrate 12. Accordingly, the signal input to the motor control circuit of the control substrate 12 is superimposed with the external noise of high frequency as indicated by a signal waveform of (d) in FIG. 13. As just described, an FG signal fed back to the control circuit is influenced by external noise because, in the motor driving device shown in FIG. 12, the motor drive circuit and the control circuit are arranged on separate substrates in view of miniaturization and cost reduction, and thus the noise picked up on the transmission path between the circuits is superimposed on the FG signal that is being transmitted through the transmission path.
The FG filter 4 is arranged on the control substrate 12 to filter signals and remove such noise, whereby the FG sensor output from which noise is removed is output to the ASIC 11. In FIG. 13, (e) indicates the digital FG signal output after passing through the FG filter 4. The conventional motor driving device disclosed in Japanese Patent Application Laid-Open No. 2006-6066 does not include such a noise removing unit; however, it is herein assumed that the motor driving device shown in FIG. 12 includes a removing unit such as a noise filter.
In the ASIC 11, a timer 7 generates a frequency signal corresponding to a specified target rotation speed of the main motor 1. A comparing unit 5 compares the frequency signal with the digital FG signal fed back from the drive substrate 13. Specifically, the comparing unit 5 compares rotation frequency (FG) signals from the timer 7 and the FG sensor 14, and generates a correction signal for adjusting the actual rotation speed of the main motor 1 to the target speed. An excitation signal generating unit 6 determines a phase excitation signal with respect to each phase of the main motor 1 based on the correction signal, and outputs the phase excitation signal to the drive substrate 13 as a control signal. The phase excitation signal is, for example, a PWM signal indicating excitation timing.
A motor driver 10 on the drive substrate 13 corresponds to a supply unit that supplies current to excite the main motor 1, and supplies the main motor 1 with current based on the phase excitation signal input from the ASIC 11 on the control substrate 12. Thus, the main motor 1 rotates at the target speed.
Ideally, in the control operation of the motor driving device explained above, the FG sensor output to be fed back is digitally converted so that its rising edge coincides with the time at which the output voltage becomes 0. However, time delay occurs in the rising edge depending on the threshold voltage of the Schmitt comparator 3 indicated by (b) and (c) in FIG. 13, and the minimum value of the high level input voltage of the ASIC 11 indicated by (e) in FIG. 13. If the time delay is large, when there are fluctuations in speed of the main motor 1 due to load fluctuations or external factors, the ASIC 11 requires a longer time to recognize the speed fluctuation. Accordingly, the time taken to correct the actual rotation speed to the target speed is prolonged, and the rotation accuracy degrades.